The objective of the proposed research is to identify and characterize the mechanisms responsible for passive and active intestinal transport of bile acids. A thermodynamic analysis of bile acid transport should provide important information on passive permeation, i.e. 1) the change in energy required for transport from water into highly purified jejunal brush border membrane vesicles following the addition or deletion of a particular substituent group on the bile acid molecule and 2) the relative polarity of these intestinal mucosal membranes compared to other biological or artificial membranes. For an understanding of active transport, studies are designed to: 1) characterize ionic gradients and an electrical potential that facilititate the accumulation of bile acids against their electrochemical gradient in ileal brush border membrane vesicles, 2) correlate basolateral membrane Na-K ATPase activity and ileal bile acid transport in vitro and 3) isolate a specific receptor on the ileal brush border membrane involved in active bile acid transport. The methods of procedure represent the sequential breakdown of the small intestine from in vivo experiments to progressively smaller functional components that are more intimately involved in bile acid transport. In these studies the transport and binding kinetics and characteristics for jejunum and ileum are compared at each level and between levels of tissue, cellular and subcellular isolation. These experiments should provide not only greater understanding of the controlling mechanisms for bile acid intestinal transport but also insight into the unresolved question of how translocation across membranes is achieved for all transport systems.